Changes In Intangible Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Finite Lived Intangible Assets [Line Items]
Gross carrying amount	$ 124,655	$ 124,655
Accumulated amortization	107,445	104,398
Net carrying amount	17,210	20,257
Core deposit intangible assets [Member]
Finite Lived Intangible Assets [Line Items]
Gross carrying amount	50,059	50,059
Accumulated amortization	43,728	42,209
Net carrying amount	6,331	7,850
Customer relationships [Member]
Finite Lived Intangible Assets [Line Items]
Gross carrying amount	71,342	71,342
Accumulated amortization	60,463	58,935
Net carrying amount	10,879	12,407
Other intangible assets [Member]
Finite Lived Intangible Assets [Line Items]
Gross carrying amount	3,254	3,254
Accumulated amortization	$ 3,254	$ 3,254